The present invention pertains to the art of high-voltage power supplies. It finds particular application in conjunction with high power generators for CT scanners and will be described with particular reference thereto. However, it is to be appreciated, that the present invention will also find application in conjunction with high-voltage supplies for other purposes.
Early x-ray tubes were provided with oil-filled transformers for providing an unregulated, alternating current source of high-voltage power. The tube itself acted as a rectifier, emitting radiation on alternating half cycles when the anode was positive and the cathode was negative. Subsequently, diode rectifier tubes, filter capacitors, and controlled grid tubes were added to deliver smoother and more stable power, improving image quality and repeatability. By operating at a 60 Hz power line frequency, these x-ray generators were characterized by their large size, heavy weight, and high stored energy. They were also reliable, had a low cost, were well understood, and were relatively simple to manufacture. Of course, they suffered from significant 60 Hz x-ray output fluctuations.
Another type of power supply developed for commercial use was a solid-state switching-type high-voltage power supply. These power supplies incorporated a kilohertz range inverter which reduced the size and weight of an HV transformer and output filter. Kilohertz range ripple had serious detrimental effects, particularly in sensitive x-ray equipment like CT scanners which measure x-ray variation at the detectors with high sampling rates to generate a diagnostic image. To smooth the ripple, capacitors were added at the output. The capacitors stored energy which slowed switching response and contributed to arcing problems. The capacitance emptied its stored energy into the short circuit caused by the arcing increasing anode and other tube damage. These switching power supply generators were also plagued by numerous problems due to their complexity and dependence on SCR inverters, infamous for their commutation failures.
Individual transformers commonly were used to boost line voltage to a few thousand volts. Stacked voltage multipliers, for example, were used to increase the voltage to the +75,000 and -75,000 volt levels commonly applied across today's x-ray tubes. Pairs of diodes or diode bridges or half bridges were connected by capacitors. The current pulses built voltages on the capacitors. A sufficiently large number of diodes and capacitors were connected in series that the voltage at the end had built to about 75,000 volts.
In a cascade arrangement, each transformer had a capacitance connected across its output to act as a voltage source at the voltage level of the transformer output. A sufficient number of the capacitors were connected in series to build the voltage to 75,000 volts or other selected voltage level. The stack of capacitors stored a large amount of energy.
The present invention provides a new and improved high-voltage power supply particularly adapted for x-ray tubes which overcome the above-referenced problems and others.